Vitamin A

What it does: Vitamin A is an antioxidant, and it’s also essential to maintaining low light and color vision, mucus membranes (which help to protect your body from disease), and skin cells.

Why you might need it: People with higher levels of vitamin A report a lower risk of lung cancer, though the benefits didn’t extend to supplements—only to those who made foods with vitamin A an integral part of their diet.

Where to get it: Kale, carrots, spinach, and sweet potatoes are great sources of beta-carotene, which the body converts into vitamin A. Most multivitamins also contain vitamin A, or you can take it on its own. Too much can be toxic, and guidelines by the Institute of Medicine say not to supplement with more than 10,000 IU per day.

Acetyl L-carnitine

What it does: Acetyl L-carnitine is a form of the amino acid L-carnitine, which moves fatty acids into the mitochondria of your cells—particularly muscle cells—to be burned for energy, and then moves the waste generated by that reaction back out to be discarded.

Why you might need it: It heals your brain and improves metabolism. An Italian study found that taking acetyl L-carnitine reduced depressive symptoms in alcoholics, and a study at the University of Massachusetts found that it improved memory and cognitive performance in older adults. (For more ways to think faster and clearer, try these 27 Ways to Power Up Your Brain.) Acetyl L-carnitine may also protect you from heart attacks, according to a separate study from Italy. Moreover, taking L-carnitine has been shown to improve the health of your sperm and even reduce insulin sensitivity.

Where to get it: The L-carnitine from red meat is the easiest form for your body to digest and use—between 54 and 86 percent is made available to your body, according to the Linus Pauling Institute. In fact, one 8-ounce steak contains between 100 and 320 milligrams. On the other hand, you only absorb about 15 to 18 percent of the L-carnitine from oral supplements. To boost your body's levels, you can take 1,000 mg to 2,000 mg per day. More than 3,000 mg daily may cause nausea, vomiting, and diarrhea.

$35 for 100 ct. of 500 mg, Amazon.com

Beta-Carotene

What it does: Beta-carotene gives carrots and other red and orange vegetables their color. It functions as an antioxidant, scavenging free radicals throughout the body. It's also converted into vitamin A. Fun fact: It also has this bizarre (and beneficial) affect on your skin.

Why you might need it: While vitamin A, as a product of beta-carotene, is an essential nutrient, supplementing with high doses has been shown to increase the risk of lung cancer and prostate cancer, according to the National Institutes of Health.

Where to get it: Carrots and sweet potatoes, along with kale, spinach, collard greens, and other leafy vegetables. Because of safety concerns, and since there are so many natural sources of beta-carotene, it's usually not recommended as a supplement, says Stephen Dahmer, M.D., a physician at the Center for Health and Healing at Beth Israel Medical Center in New York.

Boron

What it does: Boron is not well-studied, but it seems to help the body process other minerals and nutrients. It's also believed to maintain healthy bones.

Why you might need it: One study found that taking 10 mg of boron increased testosterone and decreased estradiol, a form of estrogen, within just a few hours, along with increasing levels of vitamin D in the blood. But there's little evidence to support the notion that boron supplementation can help with chronic conditions. (Click here to learn how you can boost your testosterone the natural way.)

Where to get it: Boron is found in almonds and other nuts, kidney beans, avocado, and raisins.

$6 for 100 ct. of 300 mg, vitaminshoppe.com

Butterbur

What it does: This herb contains compounds that block leukotrines, which are signaling molecules that cause contractions in your throat during an allergic reaction. By blocking the leukotrines, butterbur helps relieve those allergic symptoms.

Why you might need it: In a Swiss study, taking butterbur was just as effective as cetrizine, the active ingredient in Zyrtec, for quelling allergy symptoms. The catch? Patients had to take four butterbur tablets a day versus one cetrizine pill. Other studies have found that taking 150 mg of butterbur daily reduced the frequency of migraines.

Where to get it: If you're allergic to ragweed, chrysanthemums, marigolds, and daisies, taking Butterbur may trigger a reaction. Otherwise, follow the directions on the label.

$8 for 60 ct. of 75 mg, amazon.com

Vitamin C

What it does: Vitamin C is an immune system booster, preventing illness and infections in wounds. It also converts inactive folic acid into the active form and plays a role in the formation of hemoglobin, the protein that carries oxygen in red blood cells.

Why you might need it: Studies show that taking vitamin C can reduce the duration of a cold by almost two days. It can also reduce the frequency of exercise-induced asthma, and prevent upper respiratory infections in people who are training for marathons and other endurance races. A new study also shows that vitamin C can have beneficial brain effects. (Click here to find out what vitamin C can do for your eyesight, too—you won't believe what researchers found!)

Where to get it: Vitamin C should be in your multivitamin, in which case you'll want at least 200 mg daily. “If you get a cold, up your dosage to between 1,000 and 2,000 a day, but only for as long as you're sick,” says Stephen Dahmer, M.D., a physician at the Center for Health and Healing at Beth Israel Medical Center in New York.

$30 for 300 ct. of 1,000 mg, amazon.com

Calcium

What it does: Calcium is a very important mineral in the body, maintaining bone strength and allowing the nervous system to relay messages. It also plays a part in releasing hormones and enzymes that provide a wide array of biological functions.

Why you might need it: A Canadian study found that people who took 1,200 mg of calcium supplements over a 15-week weight-loss program lost 14 more pounds than those who didn't take enough calcium. “Unless you have problems digesting dairy, most men don't need to supplement with calcium,” says Stephen Dahmer, M.D., a physician at the Center for Health and Healing at Beth Israel Medical Center in New York.

Where to get it: Dairy products are how most people get calcium—three servings of milk or yogurt a day is enough to get what your body needs. It's also found in spinach and other leafy greens. If you do take calcium supplements, 2,500 mg per day is the upper safe limit for adults, according to the National Institutes of Health.

$20 for 300 ct. of 250 mg, vitaminshoppe.com

Chromium Picolinate

What it does: Chromium is known to be a necessary element in the human body, but scientists are still analyzing its precise functions. It's thought to play a role in how your body metabolizes protein, carbs, and fat.

Why you might need it: Chromium picolinate was thought to help with insulin sensitivity, glucose tolerance, and preventing diabetes, but a six-month randomized controlled trial at Yale University didn’t find any benefits. In a separate study, those same researchers found that chromium picolinate supplementation also didn't help with weight loss. However, some research has indicated that chromium picolinate could help Alzheimer's patients improve memory and cognitive function.

Where to get it: The National Institutes of Health recommends taking 200 micrograms a day for therapeutic purposes.

$15 for 250 ct. of 200 micrograms, amazon.com

Coenzyme Q10 (Co Q10)

What it does: Coenzyme Q10 has several functions in the body, including helping the mitochondria within your cells produce energy and neutralizing free radicals as an antioxidant.

Why you might need it: To improve your interval workouts. A Japanese study found that cyclists who supplemented with 300 mg of Co Q10 for eight days increased their maximum speed during 10-second sprints, and felt less fatigue afterward. Supplementing with Co Q10 has also been shown to reduce inflammation and free radicals created by bouts of heavy exercise. (For more fitness tips and tricks, sign up for our free Exercise of the Week newsletter.) It's also sometimes prescribed to recent heart attack victims to prevent a second cardiac event, and it may reduce low-density lipoprotein levels.

Where to get it: Take 300 mg daily, with food. Co Q10 may lower blood sugar and decrease blood pressure, so talk to your doctor before you start taking it if you're diabetic or on blood pressure medication.

$40 for 72 ct. of 100 mg, amazon.com

Coleus Forskohlii

What it does: Coleus forskohlii is produced from the roots of a tropical plant. In the body, it activates an enzyme called cyclic adenosine monophosphate, which increases your metabolism and the utilization of body fat.

Why you might need it: Research is fairly limited on coleus forskohlii. One study at the University of Kansas found that taking it for 12 weeks increased free testosterone and lean body mass, while decreasing body fat in men.

Where to get it: You'll usually see coleus forskohlii, or its derivative forskolin, combined with other compounds believed to help cut fat in thermogenics and weight-loss supplements. Shoot for 250 mg twice a day.

$18 for 60 ct. of 325 mg, vitaminshoppe.com

Conjugated Linoleic Acid

What it does: Conjugated linoleic acids are a family of similar fatty acids. CLA was discovered thanks to its anti-cancer properties; it also functions as a powerful antioxidant.

Why you might need it: To lose body fat. CLA seems to enhance the fat-burning effects of exercise, along with increasing your resting metabolic rate, and preventing your body from storing fat. Supplementing with CLA has been shown to reduce body fat and maintain lean mass, and appears to be particularly effective if it’s taken as part of an exercise regimen. A Japanese study found that CLA helped people reduce fat specifically around their waist and hips.

Where to get it: CLA is found naturally in meat and dairy products. You can take between 1 and 3 grams per day.

$38 for 120 ct. of 770 mg, amazon.com

Creatine

What it does: Creatine helps reload your muscles after they break down adenosine triphosphate into adenosine diphosphate for energy. The end result from taking a creatine supplement is improved endurance in workouts, allowing you to hang in for more reps and sets.

Why you might need it: In just 30 seconds of high-intensity exercise, you use up as much as 80 percent of the creatine phosphate stored in your muscles. One study in the Journal of Strength and Conditioning Research found that men who supplemented with creatine as part of a weight-lifting regimen for six weeks improved their leg-press strength by 62 percent. (Sign up for the free Men's Health Personal Trainer newsletter for more muscle-building tips delivered to your inbox every week!)

Where to get it: There are many forms of creatine on the market, but it's best to stick with the most-studied version: creatine monohydrate. Load your muscles by taking either 5 grams a day for 5 days, or 3 grams a day for 28 days. Then you can maintain creatine levels by taking 1 gram after each workout.

$19 for 100 ct. of 2500 mg, amazon.com

Vitamin D

What it does: The main role of vitamin D is to aid the body with absorbing calcium, and helping you to both build stronger bones and prevent bone loss.

Why you might need it: A study at Boston University estimated that 36 percent of otherwise healthy adults—and 57 percent of people who go to the doctor for one reason or another—are deficient in vitamin D. Why is that bad? Some studies have indicated that a lack of vitamin D may be connected to weight gain and depression. (Click here to read the latest research on vitamin D and depression.) Vitamin D may also help prevent cancer and high-blood pressure, and men with higher levels of vitamin D also produce stronger sperm.

Where to get it: As little as 10 minutes of bright sunlight—specifically, UVB radiation from the sun—prompts your skin to produce vitamin D. But since you probably spend more time basking in your office's florescent lights, it's a good idea to supplement with 2,000 IU a day, says Stephen Dahmer, M.D., a physician at the Center for Health and Healing at Beth Israel Medical Center in New York.

$11 for 300 ct. of 1000 IU, amazon.com

Vitamin E

What it does: Vitamin E's main role is as an antioxidant, scavenging free radicals in the body and reducing inflammation.

Why you might need it: People over the age of 55 who took a combination of vitamin E, vitamin C, beta-carotene, and zinc daily reduced their chances of developing macular degeneration, which leads to blindness, by 35 percent. What's more, in a Finnish study, smokers that took vitamin E were 32 percent less likely to develop prostate cancer, even though taking vitamin E doesn't seem to help nonsmokers prevent prostate cancer. (What's your prostate cancer risk? Take this 5-minute quiz to find out.)

Where to get it: Take 200 mg daily, in the form of d-alpha tocopheryl.

$32 for 225 count of 400 IU, amazon.com

Echinacea

What it does: Echinacea is a flower that has traditionally been used as a cold remedy by native Americans, and one study found it is used by as many as 20 percent of herbal medicine adherents.

Why you might need it: Some studies have found that taking echinacea was effective in helping to treat upper-respiratory infections. But when 713 people in a University of Wisconsin study took echinacea during a cold, they only cut the duration of their cold by an average of a half a day as compared to a placebo—not enough for scientists to conclude that the echinacea was working.

Where to get it: Echinacea is prepared several different ways: either as a tea, a concentrated extract, or as a juice. Most studies have examined dosages between 600 and 625 mg. It may set off an allergic reaction if you're also allergic to ragweed, marigolds, and daisies, according to the National Institutes of Health.

$16 for 100 ct. of 400 mg, amazon.com

Elderberry Extract

What it does: Treats cold and flu symptoms with antiviral properties by increasing the production of proteins called cytokines that respond to infection and inhibit the spread of viruses.

Why you might need it: To calm a raging cold or flu. One preliminary study found 28 percent of patients experiencing flu-like symptoms that took an elderberry lozenge were symptom-free after 48 hours; 60 percent got relief from some symptoms (in contrast, the placebo group’s symptoms didn’t change or worsen). Other research indicates elderberry is effective in killing bacterial infections that can lead to strep throat and skin infections. Animal studies also show that elderberry could reduce high blood cholesterol levels because it’s rich in heart-protective antioxidants called anthocyanins. Elderberry is also being studied for its potential to fight prostate cancer and gallbladder cancer.

Feverfew

What it does: The herb may be effective in treating inflammatory conditions like headaches, rheumatoid arthritis, allergies, and asthma.

Why you might need it: To snuff the pain from a head-pounder. A German study found that patients taking feverfew for 16 weeks reported the frequency of headaches decreased by 2 migraines per month in the feverfew group compared to 1.3 in the placebo group. New research suggests taking a combination of feverfew and ginger extracts relieved a headache in two hours for 63 percent of subjects—compared to 39 percent taking a placebo. Additionally, outdoor athletes can seek sun relief from feverfew: applying products that contain the herb can protect skin from UV damage, according to a study commissioned by Johnson & Johnson.

Where to get it: GNC Herbal Plus Standardized Feverfew, $18, gnc.com

Fiber

What it does: Dietary fiber adds bulk to your meal to help you feel full. There are two types: soluble and insoluble fiber. The former slows digestion and can help lower cholesterol, the latter “sweeps” your digestive system to keep things moving comfortably along.

Why you might need it: “Fiber is important for healthy digestion and proper absorption of nutrients,” says Susan Dopart, RD, author of A Recipe for Life by the Doctor’s Dietitian. New research in the journal PLoS One found that high fiber intakes were associated with lower incidence of stroke in men. People who consume lower intakes of fiber over their lifetimes also have a higher rate of arterial stiffness (a risk factor for heart disease) than those who consumed more. Results from a recent study even suggest that fiber may boost longevity—men eating the most fiber had a 23 percent reduced risk of dying. Aim for 30 grams a day. Whole food sources are best, and ground flaxseed can help you bulk up a diet that’s lacking; two tablespoons contain 3 grams.

Folate & B12

What it does: Both B vitamins, folate and B12 are involved in the production of red blood cells; folate is also a key player in building DNA, while vitamin B12 is vital in nerve cell development.

Why you might need it: To avoid a heart attack. Inadequate intake of both folate and B12 have been linked to elevated levels in the bloodstream of the amino acid homocysteine, which is believed to increase heart disease risk. And while the two nutrients work together, they also provide unique benefits. Case in point: A higher folate intake from your diet (folate is found naturally in foods like spinach and citrus fruits) may reduce the risk of colon cancer, according to a 2011 study in the American Journal of Clinical Nutrition. Meanwhile, B12 keeps you sharp: A recent study at Rush University Medical Center in Chicago found that people deficient in vitamin B12 were more likely to suffer from cognitive impairment, possibly because homocysteine also damages the brain. However, men should get folate from their diet rather than a supplement (see “folic acid” for more information), says Susan Dopart, RD, a Los Angeles-based dietician.

Folic Acid

What it does: Folic acid is a water-soluble b-complex vitamin, which the body uses to make energy-supplying red blood cells in the body. The natural form of this B vitamin, called folate, is found in food. Folic acid is a synthetic supplement.

Why you might need it: You might not. First, the good news: a 2012 study in the American Journal of Clinical Nutrition found that young men and women with the highest folic acid intake experienced a 52 percent reduced risk of high blood pressure 20 years later compared to those consuming lower levels. However, while getting enough folic acid is good, taking too much may have a downside. In a recent review by Norwegian researchers, folic acid supplements were linked to a 24 percent increase in prostate cancer risk for men. Some research suggests there may be a benefit to removing folic acid from supplements designed for men. Because folic acid is already abundant in fortified cereals and breads, men are likely getting ample amounts anyway, and taking an additional supplement can be risky. Most multivitamin supplements contain 400 mcg of folic acid, which men don’t need. If yours does, pop the multi only four times per week, says Susan Dopert, RD, a Los Angeles-based dietician.

Where to get it: Check with your doc first

Ginger Root Extract

What it does: With potent antioxidants like gingerol, the herb acts as an anti-inflammatory.

Why you might need it: Ginger is credited with easing an upset stomach, lessening motion sickness, and relieving nausea. Not only can it help ease discomfort associated with osteoarthritis and joint conditions, but research in The Journal of Pain revealed that people who took a half teaspoon of ginger days before a hard workout had a 25 percent reduction in muscle pain compared to those who received a placebo. Plus, the stuff can turbocharge your ability to fight the flu, says Sharon Richter, RD, a dietician in New York. “In general, exercise temporarily breaks down our immunity, especially after a strenuous session, but ginger can help bolster your body’s defenses,” she says. Ginger could also protect against certain cancers. In a recent study, people took two grams of ginger root or a placebo for 28 days. Those taking ginger experienced lower markers of gut inflammation, suggesting ginger may lower the risk of colon cancer.

Ginkgo Biloba

What it does: The herbal medication, packed with antioxidants (specifically flavonoids and terpenoids), not only neutralizes free radicals (which age your skin), but also dilates blood vessels and breaks up blood platelets, improving circulation.

Why you might need it: While results are inconsistent, some studies show that gingko biloba boosts brain function and may enhance memory. Swiss research indicates that people who took three 90 mg ginkgo extract tablets twice a day for 30 days improved circulation in the liver and increased the body’s production of a natural protective molecule called glutathione, which helps you get rid of free radicals. (High levels of glutathione are also associated with greater immunity, can decrease muscle damage, and improve strength and endurance in athletes.) Additionally, one recent Texas Tech study found that gingko extract helped protect the brain after a stroke.

Where to get it: Nature Made Ginkgo Biloba, $12, walgreens.com

Glucosamine and Chondroitin

What it does: Both are essential components of cartilage. Glucosamine is believed to help the body regulate painful inflammation and stimulate new cartilage formation in joints to treat osteoarthritis. It’s usually taken with chondroitin, thought to inhibit the enzymes that break down cartilage.

Why you might need it: For relief from painful osteoarthritis, a condition where the cushioning cartilage between bones wears away. Results in clinical trials have been mixed. For years, studies have looked at whether glucosamine and chondroitin could treat knee osteoarthritis and prevent cartilage from wearing down. In a 2010 University of Utah School of Medicine study, researchers concluded that taking 500 mg of glucosamine three times a day; 400 mg of chondroitin three times daily; or a combo of glucosamine and chondroitin was no better than a placebo in relieving osteoarthritis pain after 24 months. “Some people do swear by the supplement, but it’s expensive and most people I’ve seen in my practice don’t notice it makes much of a difference,” says Susan Dopart, RD, a Los Angeles-based dietician.

Where to get it: Vitamin Shoppe Glucosamine Chondroitin, $32, vitaminshoppe.com

Glutamine

What it does: An abundant amino acid in the body that stimulates protein synthesis (muscle growth). Also critical for immune, brain, and digestive function.

Why you might need it: Heavy exercise depletes the amount of glutamine in the body, impacting workout performance and stifling immunity. One study on physically active males who were mildly dehydrated found that combining L-glutamine and L-Alanine (another amino acid) increased subjects’ stamina during an exercise test, likely because the combo improves electrolyte and water absorption during exercise. Glutamine may also up your game: A recent study in the Journal of the International Society of Sports Nutrition found that basketball players who drank a combination of water and a low-dose alanine-glutamine supplement (1 g per 500 ml) did 11 percent better in a basketball-shooting performance test compared to a group that ingested only water.

Green Tea

What it does: The active ingredient epigallocatechin gallate (EGCG) is a powerful antioxidant that wipes out free radicals.

Why you might need it: To stay young. In a three-year study on adults over 65, only 7 percent of green tea drinkers downing at least five cups a day ever became disabled, compared to 13 percent of adults who drank less than one cup a day, according to the American Journal of Clinical Nutrition. Green tea is also linked to lower cholesterol and triglycerides (both factors in heart disease) and can protect against certain cancers. Furthermore, the brew can fire up your fat-burning engines and boost your metabolism: Men who supplemented a moderately intense workout with three capsules of green tea extract bumped their fat burn by 17 percent, according to a British study.

Griffonia Simplifica Seed Extract (5-HTP)

What it does: The amino acid 5-Hydroxytryptophan (5-HTP) is converted into serotonin, a mood-regulating neurotransmitter.

Why you might need it: 5-HTP has been used for decades to treat depression, insomnia, and chronic headaches. Because the supplement increases serotonin, it may help level out moods and even treat mild to moderate depression. (Click here for 5 All-Natural Mood-Boosters.) A 2011 study in the Journal of Affective Disorders concluded the supplement shows promise as a natural treatment for depression, but more research is needed. Other studies show that taking 5-HTP boosts the production of happiness- and sleep-promoting neurotransmitters and hormones like melatonin, dopamine, and norepinephrine. Taking 200-400 mg of 5-HTP at night may help you sleep more soundly.

Where to get it: Natrol 5-HTP, $20, natrol.com

Guarana Seed

What it does: Guarana, a seed found in the Amazon, is packed with caffeine—about four times the amount in coffee (which is why it so often has a starring role in energy drinks) as well as theobromine and theophylline, chemicals that stimulate the central nervous system.

Why you might need it: To stay fit in middle age. Guarana is a known appetite suppressant. In a Brazilian study, men who drank Guarana regularly had smaller waist circumferences and a lower prevalence of hypertension, obesity, and metabolic syndrome. Guarana also may help reduce your risk of blood clots, heart attacks, and strokes, and was shown to improve energy levels in cancer patients suffering from fatigue. Plus, if a hit of caffeine is often what pumps up your workout, guarana could act as a healthy substitute.

Hawthorne

What it does: A longtime alternative remedy for heart disease, the shrub contains antioxidants called oligomeric procyandins and quercetin, thought to relax blood vessels and improve circulation.

Why you might need it: To boost heart health and improve high and low blood pressure. According to a review by German researchers, the supplement may help improve the heart’s ability to pump blood and protect blood vessels from damage. That makes hawthorne potentially useful in treating atherosclerosis and heart disease. In another study published in the journal PLoS One, hawthorn extract was shown to increase the function inside the lining of blood vessels, which keeps arteries flexible. And in diabetes patients, taking 1,200 mg of hawthorn extract did a better job of lowering blood pressure than a placebo.

Where to get it: Nature’s Way Hawthorn, $10, amazon.com

Huperzine-A

What it does: Sharpens your mind. Extracted from a type of Chinese moss, Huperzine-A boosts levels of acetylcholine, a brain chemical thought to improve critical thinking, learning, and memory.

Why you might need it: A recent clinical trial found evidence that 400 micrograms boosted brainpower in patients with mild to moderate cases of Alzheimer’s disease. And a Chinese study showed that 200 micrograms helped patients with another type of brain ailment, vascular dementia, with no serious side effects. Skip it if you have epilepsy, ulcers, blockages in your digestive tract, asthma, or other respiratory conditions. Try exercise instead—it’s a proven way to ward off Alzheimer’s disease.

Iron

What it does: Replenishes your stores of this essential nutrient. Your body needs iron to produce critical proteins like hemoglobin and myoglobin, which carry the oxygen in your blood cells.

Why you might need it: Most guys get enough iron from a healthy diet, but if you’re a distance runner, a vegetarian, have restless leg syndrome, or find yourself getting unusually tired, ask your doctor for a blood test with an iron panel. And if your levels are normal, don’t bother. “If you already have enough, extra iron causes oxidative cell damage, harming more than it helps,” says Julie Chen, M.D., a physician based in San Jose, Calif. Instead, eat plenty of lean red meat (don’t worry, it won’t kill you), eggs, beans, whole grains, and leafy greens.

Where to get it: Bifera Iron Supplement, 22 mg, $17 for 30 tablets

Juniper

What it does: The berries from this common herb have been used for years in Turkish folk medicine for gastrointestinal and respiratory ailments. Recent research suggests they have antioxidant, anti-inflammatory, and germ-fighting powers, even battling staph infections.

Why you might need it: To speed wound healing and ward off urinary tract and other infections. (Of course, there are other, more fun ways to boost immunity, including getting a massage and even getting yourself off.) Make sure you drink plenty of water, since juniper berries act as a diuretic—which means more trips to the men’s room. And don’t take it for longer than four weeks, or your risk of kidney damage increases.

Where to get it: Nature’s Way Juniper Berries, 850 mg, $10 for 100 capsules

K Vitamin

What it does: K is crucial for getting your blood to clot properly; its name comes from the German word koagulationsvitamin, which sounds a lot like the word “coagulate.” New research, including a meta-analysis in the Journal of Bone and Mineral Metabolism, has found it also strengthens bones. And a few small studies have found that it may slow the growth of cancer cells.

Why you might need it: If you don’t eat many leafy greens, you’re liable to come up short of the 120 micrograms recommended for men—in fact, three of four Americans miss the mark. But don’t start taking it without your doctor’s approval. “You don’t want to mess with your equilibrium on clotting issues,” says Julie Chen, M.D., an integrative physician based in San Jose, Calif. Instead, try piling your plate with broccoli, Brussels sprouts, and kale (you can even make crispy chips from this unassuming green). Steer clear if you have liver or kidney issues, or are taking coumadin or other blood-thinning medications.

Where to get it: The Vitamin Shoppe Vitamin K-1, 100 micrograms, $5 for 100 tablets

Kava Kava

What it does: Studies back its effectiveness against anxiety, but the Food and Drug Administration warns against its use because of the risk of liver damage. What’s more, a few case reports link it to rhabdomyolysis, a dangerous breakdown of muscle fibers that can harm the kidneys.

Korean Red Panax Ginseng

What it does: Ginseng color refers not to the original plant, but to how the root is prepared. Red panax ginseng is harvested after six years, steamed, then dried. The end result: Pills, extracts, and creams packed with components called ginsenosides, which may mimic the effects of testosterone and also relax your erectile tissues—a good thing, since that makes room for the blood flow that brings your member to attention.

Why you might need it: A review in the British Journal of Clinical of Pharmacology found that red panax ginseng can treat erectile dysfunction (the best dose seems to be 900 mg, three times daily). Promising new research also shows the herb may lower blood glucose in people with type-2 diabetes, but check with your doctor first, especially if you’re taking other diabetes medications.

Where to get it: Korean Red Ginseng 90c by Progena, 900 mg, $20 for 90 capsules

L-Carnitine

What it does: Helps you stay pumped. “Carnitine carries fatty acids into the mitochondria of muscle cells to be burned for energy,” says Brian Zehetner, M.S., R.D.

Why you might need it: Your liver and kidneys manufacture this amino acid; right now, there’s little evidence getting more does anything to help your muscles work better, Zehetner says. But it may be a different story for beta-alanine, a compound that becomes l-carnitine in the body, says Roger E. Adams, Ph.D, a Dallas-based trainer and nutritional consultant. A 2010 review concluded that taking 3.2 to 5.2 grams per day for four weeks or longer may increase your ability to tough it out through high-intensity workouts.

Lutein

What it does: Lutein is a heavy hitter when it comes to your eye health and you’ll rarely find it in supplements without its sidekick, zeaxanthin.

Why you might need it: According to the American Optic Association (AOA), of the 600 carotenoids found in nature (lutein and zeaxanthin are both carotenoids), they’re the only ones that get deposited in high quantities in the retina—the light-sensitive part of the eye that’s essential for vision. There, they act as antioxidants to ward off free radical damage that can impact your eyesight. A 2012 study in the British Journal of Nutrition found that people with the greatest intake of lutein and zeaxanthin were significantly less likely to develop age-related macular degeneration, the leading cause of blindness in people over 50. Another 2012 study in the British Journal of Nutrition found that people with the highest levels of lutein and zeaxanthin experienced a 42 percent reduced risk of cataracts. The AOA suggests taking supplements that deliver about 10 mg lutein and 2 mg of zeaxanthin per day.

Lycopene

What it does: Here’s one time it’s good to see red: This potent antioxidant, which gives watermelons and tomatoes their color, protects your cells and DNA from the damaging effects of free radicals.

Why you might need it: “It’s a phenomenal anticancer protective,” says Julie Chen, M.D., an integrative physician based in San Jose, Calif. Early research shows men with precancerous changes in their prostates fought off cancer by taking 4 mg twice a day. A study in the journal Atherosclerosis found that 15 mg daily helped ward off hardening of the arteries, and another showed lycopene reduced DNA damage in sperm. Still, this is one nutrient it may be best to get from food, Chen says—processed tomatoes, such as those in marinara, have the highest levels (it’s not too hard to make your own—we’ll show you how). Be careful if you already have a prostate cancer diagnosis—there’s some evidence that lycopene could make existing cancer worse.

Where to get it: Lycopene 20 mg 60 Sgels by Swanson Premium, $7

Milk Thistle

What it does: People have been taking milk thistle for close to 2,000 years to treat liver disease—and there’s evidence it works. The active component, silymarin, protects the liver cells from booze, drugs, and other toxins, and may even repair damage that’s already been done. There’s even some evidence (thus far only in animals) that milk thistle slows the growth of cancer cells.

Why you might need it: To counteract the effects of one too many frat parties, or if you take medications such as acetaminophen that can harm the liver. Very few side effects have been seen at doses up to 1,500 mg. Reconsider taking it if you’re allergic to plants like ragweed, marigolds, and daisies. (Take heart: There are other ways to protect your liver—losing weight, for instance) And check with your doctor if you take other medications, including antidepressants, statins, and painkillers. Since some are broken down by the liver, milk thistle can increase their side effects.

Where to get it: Nature’s Way Milk Thistle standardized, 175 mg, $31 for 120 capsules

Niacin

What it does: Also known as vitamin B3, niacin helps your body use fats properly and convert food into energy. Plus, it battles high cholesterol much as statins do.

Why you might need it: We can think of a few good reasons to skip statins—and niacin can cut triglyceride levels by up to half, reduce LDL cholesterol by as much as one-fourth, and boost HDL levels by close to one-third. “However, too high of a dose can lead to a serious side effect called flushing syndrome, which includes skin flushing, heart palpitations, and even vomiting,” says Roger E. Adams, Ph.D., a Dallas-based nutritional consultant. The government recommends getting 16 mg per day from foods like dairy products, fish, and lean meats; get your doctor’s OK before taking more. He or she will probably start you on low doses and then gradually increase them. Controlling cholesterol may eventually require between 100 mg and 3 grams per day.

Why you might need it: You don’t. Instead, try these proven muscle-building techniques: train with light loads and high reps, and reach for whey protein after your workout.

Omega-3 Fatty Acids

What it does: The better question may be what fish oil, the main source of these nutrients, doesn’t do. Called “essential fatty acids” because your body needs them but can’t produce them, omega-3s boost brain health, improve muscle growth, fight inflammation, and decrease stress. But their most potent use may be as a weapon against the nation’s No. 1 man-killer, heart disease. Fish oil cuts the risk of heart attack, stroke, or similar events by 10 percent and cardiac death by 9 percent, according to a recent review paper in the British Journal of Nutrition.

Why you might need it: “From heart, brain, and nerve health to gastrointestinal and joint health, most people will benefit from supplementing with omega-3 fatty acids,” says Roger E. Adams, Ph.D., a Dallas-based nutritional consultant. “However, not all omega-3 fatty acids supplements are created equal.” Look for those that contain equal amounts of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—the two main types of beneficial omega-3s—and take a total of 1 to 4 grams.

Where to get it: By eating fatty fish such as salmon, tuna, and mackerel (each 3.5-ounce serving has about 1 gram of omega-3 fatty acids). Or, take Nature Made Fish Oil, 300 mg EPA/DHA, $17 for 250 liquid softgels.

Plant Stanols

What it does: These compounds from plants, nuts, and seeds look a lot like cholesterol—so when they travel through your intestines, they keep your body from absorbing the bad stuff. They’re often added to foods like margarine, orange juice, and cereal.

Why you might need it: Two to three grams per day can decrease your LDL 5 to 10 percent, says Brian Zehetner, M.S., R.D. They’ll work even if you already take statins, according to a 2011 study in the European Journal of Pharmacology. There are few safety concerns with stanols, which have been studied for more than 50 years, though high doses may cause nausea, indigestion, and diarrhea.

Where to get it: Nature Made CholestOff, 1.8 grams, $35 for 240 caplets

Policosanol

What it does: A general term for the natural extract from plant waxes, Policosanol has been thought to lower LDL (“bad”) cholesterol and increase HDL (“good”) cholesterol. A Cuban study found that people taking it saw a 27 percent reduction in LDL cholesterol and a 17 percent increase in HDL levels.

Why you might need it: You might not. “There have been no positive studies showing any benefits in five to six years,” says Jim Backes, associate professor at Kansas University Medical Center’s School of Pharmacy and Nutrition. “When we tried to replicate those studies in the U.S. and Germany, they fell flat.” Backes’ advice: If you’re already taking the supplement and think you’re benefiting, stick with it. “You just can’t justify starting a new patient on it, though, until there are positive studies on the topic.”

Where to get it: Nature’s Life Policosanol, 60 tablets, $12

Potassium

What it does: “Potassium helps to balance sodium in your cells,” says Lydia A. L. Bazzano, M.D., assistant professor of epidemiology at Tulane University Health Sciences Center. Taking potassium pushes sodium out of your body and into urine—a process called naturesis that can lower blood pressure by getting rid of salt, she explains. It also appears to have benefits to the cells lining blood vessels and those in the pancreas that produce insulin, she adds.

Why you might need it: Potassium is one of the body’s most critical minerals because it helps keep your body’s systems in balance, Bazzano says. Low levels are associated with a higher risk of stroke and cardiovascular diseases like hypertension and even heart attacks.

Where to get it: “Any time that you can get it from your diet, that’s best,” says Jim Backes, associate professor at Kansas University Medical Center’s School of Pharmacy and Nutrition. Almonds, bananas, and spinach are all rich in potassium. And if you already have a diet with a healthy focus on fruits and veggies, you’re probably getting enough. With a supplement, it’s easier to overload, which could overwhelm your kidneys as they try to remove it, Bazzano says.

Pyruvate

What it does: Pyruvate is a form of an organic acid, and plays a role in several metabolic pathways. It's produced in the body as the end result of breaking down glucose, or blood sugar, for energy. Pyruvate also helps your muscles take up glucose from the blood, which means your muscles won't need to tap into their own energy stores during exercise.

Why you might need it: Pyruvate helps break down body fat for energy. In one study published in the journal Nutrition, people who performed regular aerobic exercise and took 6 grams per day of pyruvate for six weeks lost 13 percent more weight than people who exercised and took a placebo. Pyruvic acid is also sometimes used as a skin treatment to reduce the signs of aging.

Where to get it: Apples contain 450 mg of pyruvate, and beer and wine each contain about 75 mg per serving. You can supplement with 6 grams daily as part of an exercise and weight-loss regimen.

$18 for 90 ct. of 1000 mg, amazon.com

Quercetin

What it does: Quercetin is a flavonoid that functions as an antioxidant and anti-inflammatory.

Why you might need it: Taking quercetin seems to relieve prostate pain and swelling, according to the National Institutes of Health. Scientists also believe it reduces the risk of heart disease brought on by chronic inflammation, and can produce a small decrease in blood pressure and cholesterol. Although quercetin is found in some energy drinks, a study on ROTC cadets at the University of Georgia determined that quercetin didn't provide any extra energy or help reduce fatigue. A study at the Georgia Institute of Technology found a statistically significant improvement in cardio endurance and VO2 max, but the researchers called the difference in athletic performance “trivial and small.”

Where to get it: Quercetin is most abundant in red wine, onions, apples, green tea, and berries. As a supplement, it's safe to take up to 500 mg twice daily for up to 12 weeks, according to the National Institutes of Health.

$16 for 90 ct. of 500 mg, amazon.com

Riboflavin

What it does: Riboflavin, also known as vitamin B2, is an essential part of red blood cells, which transport oxygen throughout your body. It also helps metabolize carbs, protein, and fat for energy.

Why you might need it: To prevent migraines. Of people who took 400 mg of riboflavin, 59 percent cut the number of migraines they suffered in half. According to the National Institutes of Health, riboflavin may also prevent cataracts.

Where to get it: Take it as part of either a vitamin B-complex supplement or in your multivitamin. Aim for at least 3 mg a day, unless you're trying to stop chronic migraines, in which case 400 mg is your number. High doses aren't toxic, so don't be alarmed if it turns your urine bright green or yellow.

$8 for 100 ct. of 100 mg, amazon.com

SAM-e

What it does: SAM-e stands for “S-Adenosyl methionine,” a compound produced in the liver that is involved in several metabolic pathways.

Why you might need it: SAM-e is as effective as aspirin for relieving pain due to osteoarthritis, although it may take up to a month of regular dosing to see an effect. And a study at Harvard University found that taking up to 1,600 mg per day is as effective as some prescription antidepressants in improving mood and alleviating the symptoms of depression.

Where to get it: The National Institute of Health recommends you take 400 mg to 1,600 mg per day for depression, and 200 mg three times daily to relieve the pain associated with osteoarthritis. If you're already taking medication for either of these ailments, talk to your doctor before you start popping SAM-e.

$30 for 36 ct. of 400 mg, amazon.com

Saw Palmetto

What it does: In the past, saw palmetto supplements were used to treat symptoms of benign prostatic hyperplasia (BPH)—a condition in which the prostate becomes enlarged. A series of recent studies, however, have challenged its effectiveness. One study published in The Journal of the American Medical Association found that saw palmetto extract was no more effective than a placebo at treating urinary symptoms and other problems linked to BPH.

Why you might need it: Saw palmetto may not be effective for treating BPH, but it may help you delay the onset of male pattern baldness. According to research published in the Journal of Alternative and Complementary Medicine, when 26 balding men took 200 mg of saw palmetto daily over a 6-month time span, 60 percent saw their hairline improve.

Selenium

What it does: Selenium helps you produce glutathione peroxidase, a key part of your body's antioxidant defense system.

Why you might need it: Selenium may help destroy cancer-causing free radicals. Researchers at the University of Arizona Cancer Center monitored 1,312 people over eight years to test whether selenium could lower the incidence of skin cancer. Half of the group took 200 mcg of selenium every day, and the other half took a placebo. Although the treatment didn’t protect against basal or squamous cell carcinomas of the skin, it was linked to a 66 percent lower risk of prostate cancer, 50 percent lower risk of colorectal cancers, and nearly 40 percent lower risk of lung cancer when compared to the placebo group. The catch: Research also suggests you may need to be deficient in selenium to reap the benefits. If you already receive enough, the supplement could do more harm than good.

Where to get it: Most commonly found in Brazil nuts, tuna, and cod.

GNC Selenium 200 (200 mcg/day), $12

St. John’s Wort

What it does: It’s most commonly used to treat mild to moderate forms of major depression.

Why you might need it: It may be more effective than even standard treatments for depression. Research published in The Journal of the American Medical Association found that St. John's Wort was just as effective as standard antidepressants for treating people with mild or moderately severe depressive disorders—yet carried fewer side effects.

Where to get it: Kira St. John’s Wort (900 mg/day); $9

Stemulite

What it does: Stemulite is a proprietary blend, which means a company throws together a potpourri of supplements and gives it a name. This particular blend constitutes a mix of alpha-lipoic acid, acetyl L-carnitine, beta-glucans, serrapeptase, quercetin, melatonin, indium, and eggplant extract. Only a few of those substances have proven to be effective.

Why you might need it: The main ingredient, alpha-lipoic acid, is one of the most potent antioxidants available, according to Jonny Bowden, Ph.D., C.N.S., and author of The 150 Healthiest Foods on Earth. Some preliminary studies have shown that alpha-lipoic acid increases the uptake of creatine, making it useful for endurance and muscle-building. “Acetyl L-carnitine has also been shown to be helpful for the brain,” he says. In one UCLA study, rats taking acetyl L-carnitine and alpha-lipoic acid performed better on memory tests. The only other ingredients in Stemulite that have the support of researchers are melatonin—a supplement designed to help you sleep—and beta glucans—a nutrient that has been shown to combat cancer cells. Not one of the remaining ingredients has enough scientific backing (or presence in the pill) to provide any noticeable benefit, Bowden says. “They're tagging this potluck kind of formulation on a couple of decent supplements and very small doses of things nobody knows anything about,” Bowden says. You're better off buying melatonin, alpha-lipoic acid, and acetyl l-carnitine individually to reap sleep-inducing, antioxidant, and memory-boosting benefits respectively.

Time-release Melatonin

What it does: Melatonin, a hormone naturally produced by the pea-sized pineal gland in your brain, is an essential piece of your circadian rhythm. The time-release version of this pill causes levels of melatonin in the blood stream to steadily rise instead of dumping it in all at once. The theory is that it will help you stay asleep, but trials have yet to find time-release tablets better or more effective than standard ones.

Why you might need it: At night, the brain churns out increasing levels of melatonin, which makes you feel groggy and helps you nod off. For some people—especially travelers who experience jet lag and shift workers, though it sometimes occurs for unexplained reasons—your melatonin release may be disrupted, which can derail your ability to fall asleep. Several studies that have only looked at time-release melatonin have found that it works: A 2011 study by researchers in France found that adults with insomnia who popped time-release melatonin daily for a year reported significantly improved sleep. And when they stopped taking the supplement they didn’t suffer rebound insomnia (insomnia that comes back even worse than before you started taking it) or withdrawal symptoms (such as anxiety and nausea) that sometimes occur when you suddenly quit swallowing over-the-counter and prescription sleep aids.

What’s more, a recent study published in the journal Diabetes, Metabolic Syndrome, and Obesity: Targets and Therapy found that time-release melatonin may offer a one-two punch for type-2 diabetics with insomnia. Not only did the supplement improve sleep and reduce the number of times patients woke during the night by at least 25 percent, it also reduced blood glucose concentrations by more than 9 percent, on average. How melatonin may benefit blood sugar is still to be determined, but researchers believe the hormone may play a role in improving the body’s ability to metabolize glucose. Bonus: It didn’t interact with any of the diabetes drugs the patients were taking.

Still, Tod Cooperman, M.D., of ConsumerLab.com, says to keep in mind melatonin is a hormone and when you change one hormone in the body you often affect others. “Couples who are trying to get pregnant should avoid melatonin because it suppresses testosterone in men, which impacts sperm production, and can prevent ovulation in women,” Cooperman said.

Thiamin

What it does: The body relies on thiamin, also known as B1, to convert food into energy, so a lack of it could result in a rare condition called Beriberi, which causes severe muscle weakness and fatigue.

Why you might need it: It’s very unlikely that you’re actually deficient in thiamin, says Jonny Bowden, Ph.D., C.N.S., and author of The 150 Healthiest Foods on Earth. Plus, most multivitamins, if you take one, contain at least the recommended daily amount of 1.2 mg for men and many foods, including cereals, are fortified with it. Alcoholics may be at risk of thiamin deficiency since excessive alcohol intake may interfere with the body’s ability to absorb the vitamin, but a night of heavy drinking isn’t going to put you at risk, Bowden adds. “People who take diuretics to treat congestive heart failure could become deficient because they may cause the body to get rid of too much thiamin. If you take water pills ask your doctor if you should add a thiamin supplement.”

Toothed Clubmoss

What it does: Toothed clubmoss may conjure up images of the people-eating plant in Little Shop of Horrors, but this flora feeds the brain instead of the other way around. The active ingredient in clubmoss, called Huperzine A, inhibits production of an enzyme that breaks down acetylcholine, a substance involved in mental function and memory.

Why you might need it: You might not—at least, not for many years. When the enzyme is blocked, levels of acetylcholine in the brain rise. The result: The chemical improved memory in elderly people with forgetfulness and patients with Alzheimer’s disease, according to a clinical trial in China. A meta-analysis of six studies including more than 450 Alzheimer’s patients found that those taking Huperzine A showed greater improvements in cognitive function than those popping a placebo, reports the journal Cochrane Database of Systematic Reviews.

So could clubmoss cure forgetfulness and sharpen your mental edge? Tod Cooperman, M.D., of ConsumerLab.com, says no. “Unless you have low levels of acetylcholine in the brain, which occurs with people who have dementia or Alzheimer’s, more won’t make you smarter.” And don’t feed it to grandpa without consulting a doctor first—the supplement may interact with drugs that function in a similar fashion (such as those that treat Alzheimer’s disease) and may decrease heart rate, which could worsen preexisting cardiovascular issues.

Urtica Diocia

What it does: Also known as stinging nettle, urtica diocia has been shown to deliver several different benefits in the body. The root is most commonly associated with treating benign prostatic hyperplasia (BPH), or enlarged prostate, which causes problems like a constant urge to urinate and a weak urine stream.

Why you might need it: Research show it does a pretty darn good job at quelling BPH symptoms. A 2005 study in the Journal of Herbal Pharmacotherapy found that after six months of treatment, 81 percent of BPH patients who took urtica dioica experienced improved symptoms compared with only 16 percent of those who popped a placebo. And a German study published in 2000 reported that the plant worked equally as well as the popular BPH prescription medication finasteride (Proscar) at controlling symptoms. More research is needed to understand how, exactly, it helps. “You’re unlikely to find stinging nettle in a supplement alone,” says Jonny Bowden, Ph.D., and author of The 150 Healthiest Foods on Earth, “but it’s in most prostate formulas along with ingredients such as saw palmetto and pumpkin seed oil, which have also been shown to benefit the prostate.” Look for those with a combination of about 160 mg saw palmetto and 120 mg urtica dioica. Nettle also may help with hay fever. A 2009 study in the journal Phytotherapy Research shows that the leaves of the plant may inhibit the formation of several symptom-causing molecules including histamine and prostaglandins. Bowden suggests downing a formula that contains 600 mg nettle.

Valerian Root

What it does: Valerian and Valium sound similar for a reason: “Valium is a highly-concentrated pharmaceutical version of valerian root,” says Jonny Bowden, Ph.D., and author of The 150 Healthiest Foods on Earth. And they work in similar ways: A 2009 study in the journal Neuropharmacology found that compounds in valerian root called valerenic acid and valerenol bind to a specific site on receptors in the brain, which reduce levels of the GABA neurotransmitter that’s associated with experiencing anxiety. “Unlike Valium, which has an immediate and powerful effect, valerian root takes a much longer time to work,” Bowden says. “You may need to take it daily for several weeks before you start to experience the effects.”

Why you might need it: Valerian root may improve sleep for insomniacs: One study found that those taking 530 mg valerian root twice a day for four weeks experienced a 30 percent boost in snooze quality compared to only a 4 percent improvement among those taking a placebo. Then again, the dried root’s dirty-socks smell could knock you out.

Where to get it: Nature’s Way, 180 caps, $10, amazon.com

Whey

What it does: Whey protein, derived from milk, is the best source of leucine, an amino acid that behaves more like a hormone in your body. It’s a key player when it comes to muscle repair and growth, appetite suppression, and weight loss.

Why you might need it: Boosting leucine levels leads to greater muscle protein synthesis—the intricate process that helps promote the remodeling, repair, and muscle growth that occurs after exercise, a 2011 study in the American Journal of Clinical Nutrition reports. In the study, military personnel rode stationary bikes for 60 minutes on two separate occasions. Both times they chugged a beverage with equal amounts of protein, but one drink contained 1.9 grams of leucine, while the other packed 3.5 grams. Researchers found protein synthesis was 33 percent higher after the larger leucine dose. A 2011 study by USDA scientists found that overweight and obese adults who downed whey protein shakes twice a day for six months lost about two pounds and an inch around their waist. Meanwhile, those who drank soy protein shakes, which contained the same amount of calories and protein as the whey, packed on a few pounds. Researchers found that the whey protein group had significantly lower blood levels of the hormone ghrelin, which spurs appetite, meaning the protein may keep you feeling fuller longer. Your move: Down about 25 grams of whey protein postworkout.

Zinc

What it does: You can count on zinc for two things: Treating colds and producing sperm. While it won’t keep an upper respiratory infection at bay, it’ll cut down on the number of days you feel under the weather.

Why you might need it: A 2011 study found that taking more than 75 mg of zinc per day reduced the duration of colds by 42 percent. “In order for it to work you need to take it at the first sign of a cold, such as a sore throat, and take it in lozenge form multiple times per day according to the package’s directions,” says Tod Cooperman, M.D., of ConsumerLab.com. “Zinc works locally in the throat to soothe pain and halt the virus, so pills and chewable options won’t have the same effect as a lozenge that coats the throat.” Since your body relies on zinc to manufacture sperm, you’ll want to be extra vigilant about getting enough. Vegetarians are most at risk of being too low since zinc is primarily found in meat, poultry, and seafood (oysters are one of its richest sources). For healthy and plentiful swimmers, shop for a supplement that meets the RDA of 11 mg.

Where to get it: Nature Made Zinc, 100 tablets (30 mg), $8, amazon.com